NIP-48: Proxy Tags

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diff --git a/48.md b/48.md
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+NIP-48
+======
+
+Proxy Tags
+----------
+
+`draft` `optional` `author:alexgleason`
+
+Nostr events bridged from other protocols such as ActivityPub can link back to the source object by including a `"proxy"` tag, in the form:
+
+```
+["proxy", <id>, <protocol>]
+```
+
+Where:
+
+- `<id>` is the ID of the source object. The ID format varies depending on the protocol. The ID must be universally unique, regardless of the protocol.
+- `<protocol>` is the name of the protocol, e.g. `"activitypub"`.
+
+Clients may use this information to reconcile duplicated content bridged from other protocols, or to display a link to the source object.
+
+Proxy tags may be added to any event kind, and doing so indicates that the event did not originate on the Nostr protocol, and instead originated elsewhere on the web.
+
+### Supported protocols
+
+This list may be extended in the future.
+
+| Protocol | ID format | Example |
+| -------- | --------- | ------- |
+| `activitypub` | URL | `https://gleasonator.com/objects/9f524868-c1a0-4ee7-ad51-aaa23d68b526` |
+| `atproto` | AT URI | `at://did:plc:zhbjlbmir5dganqhueg7y4i3/app.bsky.feed.post/3jt5hlibeol2i` |
+| `rss` | URL with guid fragment | `https://soapbox.pub/rss/feed.xml#https%3A%2F%2Fsoapbox.pub%2Fblog%2Fmostr-fediverse-nostr-bridge` |
+| `web` | URL | `https://twitter.com/jack/status/20` |
+
+### Examples
+
+ActivityPub object:
+
+```json
+{
+  "kind": 1,
+  "content": "I'm vegan btw",
+  "tags": [
+    [
+      "proxy",
+      "https://gleasonator.com/objects/8f6fac53-4f66-4c6e-ac7d-92e5e78c3e79",
+      "activitypub"
+    ]
+  ],
+  "pubkey": "79c2cae114ea28a981e7559b4fe7854a473521a8d22a66bbab9fa248eb820ff6",
+  "created_at": 1691091365,
+  "id": "55920b758b9c7b17854b6e3d44e6a02a83d1cb49e1227e75a30426dea94d4cb2",
+  "sig": "a72f12c08f18e85d98fb92ae89e2fe63e48b8864c5e10fbdd5335f3c9f936397a6b0a7350efe251f8168b1601d7012d4a6d0ee6eec958067cf22a14f5a5ea579"
+}
+```
+
+### See also
+
+- [FEP-fffd: Proxy Objects](https://codeberg.org/fediverse/fep/src/branch/main/fep/fffd/fep-fffd.md)
+- [Mostr bridge](https://mostr.pub/)
\ No newline at end of file
diff --git a/README.md b/README.md
index ff12800..cfee37d 100644
--- a/README.md
+++ b/README.md
@@ -56,6 +56,7 @@ They exist to document what may be implemented by [Nostr](https://github.com/nos
 - [NIP-45: Counting results](45.md)
 - [NIP-46: Nostr Connect](46.md)
 - [NIP-47: Wallet Connect](47.md)
+- [NIP-48: Proxy Tags](48.md)
 - [NIP-50: Keywords filter](50.md)
 - [NIP-51: Lists](51.md)
 - [NIP-52: Calendar Events](52.md)
@@ -191,6 +192,7 @@ When experimenting with kinds, keep in mind the classification introduced by [NI
 | `nonce`           | random                               | --                   | [13](13.md)              |
 | `preimage`        | hash of `bolt11` invoice             | --                   | [57](57.md)              |
 | `price`           | price                                | currency, frequency  | [99](99.md)              |
+| `proxy`           | external ID                          | protocol             | [48](48.md)              |
 | `published_at`    | unix timestamp (string)              | --                   | [23](23.md)              |
 | `relay`           | relay url                            | --                   | [42](42.md)              |
 | `relays`          | relay list                           | --                   | [57](57.md)              |